The present invention generally relates to equipment-grounded high voltage systems that may experience leakage current, the means and methods by which the electrical shock hazards from fault current may be mitigated; and a system by which components of said high voltage systems may be monitored, tested, and disabled in response to leakage current such that personal injury is minimized.
It is well known that electrical systems require grounding paths to ensure correct operation of the equipment and to provide a discharge path for leakage and fault current. In addition to providing a path to discharge leakage and fault current, it is desirable for electrical systems to have means by which fault current may be detected and the electrical source disabled in response to some threshold. Ground fault circuit interrupters (GFCIs), most commonly known in use for 120 volt AC circuits and household outlets, are one such means. While GFCI devices are currently in wide use for low voltage systems and more recently higher voltage systems as add-on components, their use has not yet been made integral to equipment-grounded high voltage systems (such as those in sports lighting applications) for a variety of reasons (such as the necessary modifications to make existing GFCI technology applicable to HID lamp systems). Therefore, it is desirable to develop means and methods to provide GFCI functionality to equipment-grounded high voltage electrical systems, particularly those used in sports lighting applications or other applications which use ballast-capacitor-lamp type loads. It is further desirable for said GFCI functionality to be actively controlled and monitored such that faults are accurately identified and shutdown times per governing codes are achieved.
An equipment-grounded high voltage system that may benefit from the aforementioned GFCI functionality, such as the sports lighting system illustrated in FIG. 1A, can generally be characterized by the following.                1. A transformer 10 from the utility company provides electrical power to a service distribution cabinet 30 via a distribution wire 20.        2. Electrical power from service distribution cabinet 30 travels to a control/contactor cabinet 40 via a power line 21.                    a. Note that in subsequent figures power line 21 is shown to be three-phase, however, this is by way of example and not by way of limitation. Single-phase power line electrical systems would likewise benefit from aspects of the invention.                        3. Electrical power from control/contactor cabinet 40 travels to a pole cabinet 50 housed on each lighting support structure 60 via power line 21.        4. Electrical power at pole cabinet 50 powers one or more lamps 61, illuminating a field 70.        
Earth grounding to protect against adverse electrical effects, such as lightning, may be provided via earth grounds 80 connected to each pole cabinet 50 and may be such as is described in U.S. patent application Ser. No. 12/709,991, incorporated by reference herein. Equipment grounding may be provided via an equipment ground 81 connected to distribution cabinet 30 and via an equipment ground wire 82, and may be such as is described in U.S. patent application Ser. No. 12/559,863 incorporated by reference herein. Remote control capabilities of the electrical system may be enabled by a control center 90. Means and apparatus for remotely controlling operation of the system illustrated in FIG. 1A may be such as is described in U.S. Pat. No. 6,681,110, incorporated by reference herein, and commercially available under the trade name CONTROL-LINK® from Musco Lighting, LLC, Oskaloosa, Iowa, USA. As may be appreciated by one skilled in the art, the currently commercially available CONTROL-LINK® product may differ from that described in U.S. Pat. No. 6,681,110 as the mode of communication between an onsite component and a central server discussed in said patent (e.g. analog cellular signal) may comprise alternate modes of communication (e.g. satellite, Global System for Mobile communications (GSM), Code Division Multiple Access (CDMA), etc.).
As can be seen in FIG. 1A, if fault conditions due to fault current arose in one part of the system, for example at lamp 61 of Pole A, the condition could only be isolated by shutting down the entire electrical system manually at service distribution cabinet 30, remotely by control center 90, or manually at pole cabinet 50 (provided it was safe to do so, which may not be the case due to electrical shock hazards). There is no GFCI-type functionality that would allow lamp 61 to shut down while the rest of the electrical system operated normally. Further, there is no monitoring functionality of the leakage current to determine if it was safe to manually remove power at pole cabinet 50 controlling lamp 61.
Underwriters Laboratories, Inc. (UL) set forth requirements for providing GFCI functionality for systems such as that illustrated in FIG. 1A, however, use of currently available GFCI devices is limited by the lack of prevention of false detection of fault current and disabling of the electrical system, commonly referred to as nuisance tripping. High voltage systems generally have a higher leakage current than low voltage systems which may cause a GFCI device to erroneously detect a fault during normal operation. For example, UL-943 states that for systems up to 150 volt AC from any line voltage to ground (such as the aforementioned 120 volt systems), the minimum fault current to ground threshold (commonly referred to as the minimum trip point) is 6 mA, which is readily achieved with commercially available GFCI devices (whether as add-on components or as part of the circuit). For higher voltages, for example 150-300 volt phase to ground, UL-943C states the minimum trip point is 20 mA if a reliable equipment grounding system is in place; reliable grounding is defined by UL as a grounding system or double insulation system that satisfies U.S. National Electric Code (NEC) 250.110 (6) and 250.114 (2). UL-943C further states that the reliable equipment grounding system must have a ground monitoring function that will prevent more than a 150 volt voltage drop in the grounding circuit. However, for high voltage systems such as that illustrated in FIG. 1A, the 20 mA threshold may be encountered during some phases of normal operation, such as lamp 61 startup, resulting in nuisance tripping. Commercially available GFCI devices do not address this concern.
Also challenging is disabling the electrical system in the desired time constraints. For example, UL-943 states that for systems up to 150 volt AC from any line voltage to ground (such as the aforementioned 120 volt systems) the required time for shutdown at the minimum trip point is 5.59 sec, which is usually readily achieved with commercially available GFCI devices (whether as add-on components or as part of the circuit). For higher voltages, for example 150-300 volt phase to ground, UL-943C states the required time for shutdown at the minimum trip point is 1 sec if a reliable grounding system is in place. However, the required time for shutdown decreases as the trip point increases and for electrical systems carrying 150-300 volt phase to ground, the shutdown time threshold decreased to 20 msec. For conventional high voltage systems such as that illustrated in FIG. 1A, 20 msec does not allow sufficient time for detection of the fault, communication to control/contactor cabinet 40 to shutdown, and shutdown of the desired electrical component; this due in part to the amount of time required to disconnect high voltage devices such as contactors that are common to ballast-capacitor-lamp type loads. Again, commercially available GFCI devices do not address this concern.